1. Field of the Invention
The present invention relates to a magnetron, and more particularly, it relates to a magnetron in which vanes are improved in structure.
2. Description of the Prior Art
FIG. 1A is a partially fragmented front elevational view showing structure of a conventional magnetron, and FIG. 1B is a cross-sectional view taken along the line IB-IB in FIG. 1A. FIG. 1C is a cross-sectional view taken along the line IC-IC in FIG. 1B. Referring to these drawings, a magnetron 1 is provided in its center with a cathode 2, which has a filament in the interior thereof for generating electrons. A plurality of panel-shaped vanes 3 are radially arranged to surround the cathode 2. The outer end portions of these vanes 3 are fixed to the inner wall of an anode cylinder 4. A pair of strap rings 5 are fixed to each of upper and lower ends of each vane 3 as shown in FIGS. 1A and 1C, for short-circuiting every other vane 3. A cavity resonator is formed by each of spaces 6 defined by the respective adjacent vanes 3 and the inner wall of the anode cylinder 4 and partially opened toward the cathode 2, so as to determine the oscillation frequency of the magnetron by the resonance frequency of the cavity resonator. A space 7 defined between the vanes 3 and the cathode 2 is called an interaction space. An even direct-current magnetic field is applied to the interaction space 7 in parallel with the central axis of the cathode 2. To this end, permanent magnets 8 are arranged in the vicinity of the upper and lower ends of the anode cylinder 4, respectively. A direct-current or low-frequency high voltage is applied between the cathode 2 and the vanes 3.
In the aforementioned structure, a high-frequency electric field formed in the cavity resonator is concentrated to the forward end portions of the respective vanes 3, and partially leaked into the interaction space 7. An electron group 9 emitted from the cathode 2 rotatingly passes through the interaction space 7 in which the leaked high-frequency electric field and the direct current magnetic field are superposed, whereby interaction takes place between the electron group 9 and the leaked high-frequency electric field and energy of the electron group 9 is supplied to the high-frequency electric field for oscillation. Microwaves obtained by this oscillation are outwardly guided through an antenna 10 which is connected to the vanes 3. Since conversion efficiency to the microwave power, in this case, is not 100%, the energy of the electron group 9 is partially consumed as heat. Therefore, fins 11 are provided along the outer circumference of the anode cylinder 4 for radiation of the heat. It is to be noted that the internal structure of the anode cylinder 4 is shown alone and the fins 11 etc. are not shown in FIG. 1B.
It has been well known in the art that the time of the interaction between the electron group 9 generated from the cathode 2 and the leaked high-frequency electric field is increased as the amount of leakage of the high-frequency electric field is decreased, whereby the conversion efficiency to the microwave power induced in the cavity resonator from the direct input current, i.e., oscillation efficiency is improved.
Japanese Patent Laying-Open Gazette No. 161264/1979, discloses technique of improving oscillation efficiency of a magnetron by reducing leakage of a high-frequency electric field into an interaction space of the same. According to the technical idea disclosed in the subject publication, respective vanes are provided in portions between the forward and outer ends thereof with projections which are opposed with each other with intervals equal to or smaller than the intervals between opposed forward ends of the respective adjacent vanes, so as to concentrate the high-frequency electric field to the subject projections and reduce leakage of the same through the forward ends of the vanes, thereby improving the oscillation efficiency of the magnetron.
Incidentally, distribution density of the high-frequency electric field at the forward end portions of the vanes reaches the maximum at corners of the forward end surfaces of the vanes, and the same applies to even the aforementioned prior art in which merely the leakage of the high-frequency electric field is reduced. Thus, in conventional magnetrons including the aforementioned prior art, remarkable uneveness is caused in the distribution density of the high-frequency electric field in the corners and other portions of the forward ends of the vanes to disturb the interaction between the electron group and the high-frequency electric field, leading to radiation of undesired higher harmonics.